This invention relates to electronic shift control of an automatic transmission, and more particularly to a shift pressure regulator valve and diagnostic switch therefor.
In general, a motor vehicle automatic transmission includes a number of elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several speed ratios between the transmission input and output shafts. Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
The torque capacity of a clutch (on-coming or off-going) involved in a shift may be conveniently controlled by the combination of an electrically activated solenoid valve and a pressure regulator valve, as disclosed, for example, in the U.S. Pat. No. 5,911,244 to Long et al., issued on Jun. 15, 1999, assigned to the assignee of the present invention, and incorporated herein by reference. In a typical system, the solenoid valve is activated by pulse-width-modulation (PWM) at a controlled duty cycle to develop a pilot pressure for the pressure regulator valve, which in turn, supplies fluid pressure to the clutch in proportion to the solenoid duty cycle.
It is important to reliably and inexpensively diagnose clutch engagement and disengagement in the above-described clutch controls, both to verify shift progression, and to detect an inadvertent engagement or disengagement. This can be accomplished either indirectly by analyzing the transmission input and output speeds, or directly by installing pressure switches at each of the clutches. However, the diagnostic output with either of these techniques is only developed once the respective clutch has actually started to engage or disengage, which is not especially desirable if the engagement or disengagement is inadvertent. Theoretically, of course, one could alternatively measure the pilot pressure or the position of the pressure regulator valve, but such approaches are generally expensive to implement, and trouble-prone due to the characteristic dithering of a pressure regulator valve. Accordingly, what is needed is an inexpensive clutch pressure control arrangement that provides a reliable diagnostic output early in the clutch pressure control process.
The present invention is directed to an improved and self-diagnosing shift pressure regulator apparatus for an automatic shift transmission, wherein a diagnostic output provides reliable and timely diagnostic information regarding initiation and cessation of fluid supply to a friction element. According to the invention, the regulator apparatus includes a pressure regulator valve responsive to an electronically developed pilot pressure, and a diagnostic switch. The pilot pressure positions the pressure regulator valve in one of three statesxe2x80x94ON, TRIM, and OFFxe2x80x94and the diagnostic switch is continuously activated by a control pressure whenever the pressure regulator valve is in the TRIM or ON states.
The pressure regulator valve has a spool element axially positionable within a valve bore and having spaced lands defining first fluid chamber continuously coupled to the friction element, and a second fluid chamber continuously coupled to a pressure-responsive diagnostic switch. The spool element is positioned in the valve bore in response to a variable trim pressure, which is opposed by a return spring and a feedback pressure from the friction element. In the OFF state, fluid in said first and second fluid chambers is exhausted through said exhaust passages uncovered by the spaced lands, and the diagnostic switch detects a deactivated state of the valve. In the TRIM state, the valve regulates the fluid pressure in first chamber in relation to the trim pressure, the second chamber is continuously coupled to a fluid pressure passage, and the diagnostic switch detects an activated state of the valve. In the ON state, the first chamber is continuously coupled to a fluid pressure passage to fully engage the friction element, the second chamber remains continuously coupled to the fluid pressure passage, and diagnostic switch consequently continues to detect an activated state of the valve.